The more you play the more become available. I think it has something to do with the experience points (EXP) that you'll see in the top right corner of the screen. I don't know the exact levels you need to reach but you'll gradually see Managers becoming available in the list.

I think you can also use "Coins" to purchase top managers but that means investing money. I'd advse sticking it out and slowly gaining EXP for a more rewarding feeling ; )